Sequentially operable control valve for a steam turbine

ABSTRACT

A sequentially operable control valve for an axial flow steam turbine having a plurality of plugs which successively lift from their seats, the plugs and seats having surfaces which cooperatively produce consecutive throttling stages as steam passes therebetween, the first plug to lift off its seat having the greatest number of consecutive throttling stages and plugs which successively lift off their seats having a smaller number of consecutive throttling stages, the velocity through the consecutive throttling stages being slightly less than the speed of sound as the plug lifts, thus keeping the intensity of vibration and noise produced by this valve at a low level.

United States Patent [191 Meyer 51 Oct. 9, 1973 SEQUENTIALLY OPERABLECONTROL VALVE FOR A STEAM TURBINE Charles A. Meyer, Media, Pa.

Westinghouse Electric Corporation, Pittsburgh, Calif.

June 16, 1971 Inventor:

Assignee:

Filed:

Appl. No.:

References Cited UNITED STATES PATENTS 5/1967 Rankin.... 137/630.19

4/1938 Rosch 251/333 X 7/1933 Lee 251/333 X Primary Examiner-Robert G.Nilson Attorney-A. T. Stratton, et al.

5 7] ABSTRACT A sequentially operable control valve for an axial flowsteam turbine having a plurality of plugs which successively lift fromtheir seats, the plugs and seats having surfaces which cooperativelyproduce consecutive throttling stages as steam passes therebetween, thefirst plug to lift off its seat having the greatest number ofconsecutive throttling stages and plugs which successively lift offtheir seats having a smaller number of consecutive throttling stages,the velocity through the consecutive throttling stagesbeing slightlyless than the speed of sound as the plug lifts, thus keeping theintensity of vibration and noise produced by this valve at a low level.

SHEF 1UP 4 PATENTED OCT 9 I973 FIG.!

1C l m m FIG.2

1 SEQUENTIALLY OPERABLE CONTROL VALVE FOR A STEAM TURBINE BACKGROUND OFTHE INVENTION This invention relates to sequentially operable controlvalves for axial ,flow steam turbines and more particularly to such avalve having seats and plugs which cooperate to form consecutivethrottling stages across the mating seats and plugs.

With advances in technology the pressure and temperature of steamsupplied to the turbine has increased beyond the critical pressure andtemperature causing extremely high pressure drops across the controlvalves during start-up and during low load conditions. Sequentiallyoperable control valves have been used for several decades and provide agroup of relatively small diameter valves requiring relatively .smalllift forces to raise each individual valve plug from its seat, comparedto a single valve size for full load steam flow. However, as the firstvalve plug lifts off its seat it must throttle the pressure fromsupercritical pressure to partial vacuum, and such pressure dropsproduce supersonic steam velocities which cause high intensity noisesand vibratrons.

SUMMARY OF THE INVENTION In general, sequentially operable controlvalves made in accordance with this invention, comprise a plurality ofmating plugs and seats, the mating plugs and seats being disposed tosuccessively separate and thereby to allow fluid to flow therebetween.The mating plugs and seats have surfaces, which cooperate to produceconsecutive throttling stages as fluid passes therebetween. The numberof consecutive throttling stages is greatest in the first plug and seatto separate, the mating plugs and seats to successively separate havinga lesser number of consecutive throttling stages.

BRIEF DESCRIPTION OFTHE DRAWINGS FIG. 3 is similar to FIG. 2, but showsthe plug in its fully open position;

FIG. 4 is an enlarged partial sectional view of a seat and plug showinganother embodiment of this invention;

. FIG. 5 is an enlarged partial section view of a seat and plug showinga third embodiment of this invention;

FIG. 6 is a diagram showing steam flow versus pressure as the steampasses through a typical seven-stage sequentially operable control valveand showing steam flow versus pressure as steam flows through asequentially operable control valve made in accordance with thisinvention; and

FIG. 7 is a Mollier diagram showing the dissipation of energy across aplug and seat arrangement which has five throttling stages.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings.in detail, FIG. 1 shows a sequentially operable control valve'C for anaxial flow turbine T having seven separate plugs, l, 2, 3, 4, 5, 6, and7 and mating seats 1', 2', 3, 4, 5', 6, and 7, which sequentially liftto admit fluid, in this case steam, to separate segments of an annulararray of circumferentially spaced nozzles 9. The plug indicated by 1being the first plug to lift off its seat, the plug indicated by 2 beingthe second plug to lift off its seat, thus the numbers indicating thesequence in which the individual plugs lift from their seat as thecontrol valve C opens to admit more steam to the nozzles 9.

When the velocity of the fluid flowing across the seat of a valveexceeds the velocity of sound, Mach 1, i.e., the Mach number which isthe ratio of the fluid velocity to the velocity of sound in the fluid isgreater than 1.0,

objectionably high intensity noises and vibrations are produced,therefore it is desirable to provide throttling valves which will notproduce velocities in excess of Mach 1.

To provide pressure drops which produce velocities less than Mach 1 in asequentially operable control valve mating plugs and seats as shown inFIGS. 2, 3, 4 and 5, each have surfaces which cooperate to provideannular openings which increase in size in downstream direction toproduce consecutive throttling stages so disposed that the velocityacross each throttling stage remains generally equal to each other asseparation between the plugs and seats increases. The number ofthrottling stages is greater in the first plug 1 to separate from itsseat 1', as the first plug 1 to separate from its seat 1 handles thegreatest pressure drop. Plugs and seats which successively separate havea lesser number of consecutive throttling stages as they are required toprovide for smaller pressure drops. The last valve to separate from itsseat may have only a single throttling stage as the pressure drop acrossthis plug and seat is normally low enough to produce velocity less thanMach 1.

To minimize the number of consecutive throttling stages, the pressuredrop across .each stage at the instant the plug begins to separate fromits seat should be such as to produce a velocity generally equal to thevelocity of sound in the fluid, a velocity of Mach 1. The velocitiesacross the individual stages should also remain equal to each other asthe plug rises even though the velocities, on a whole, decrease due to areduction in the pressure drop.

To provide equal velocities across the stages the area of the throat oropening of the throttling stages must increase in a downstreamdirection, since the specific volume of the fluid, in this case steam,increases as the pressure decreases. To provide equal velocitiesacrossthe consecutive throttling stages the ratio of the open annulargrooves 21 and 23. The grooves 21 and 23 each have rounded protuberancesor lips 25 which extend inwardly from the upper edges of the groove. Theplug 1 has three annular skirts 27, 28 and 29. The inner skirt 27 seatson an upper surface 30 of the seat 1 and the skirts 28 and 29 registerwith the grooves 21 and 23 respectively. The skirts 28 and 29 are formedfrom frustoconical surfaces 31 and 32 and 33 and 34 each of which forman angle a with a cylindrical plane represented by the lines 35, 36, 37and 38 as shown in FIG. 3. The frustoconical surfaces are so disposedthat the angle or increases in downstream direction so that the ratio ofthe open area of the throttling stages remains generally equal as theplug lifts from the seat to provide generally equal velocities throughthe consecutive throttling stages. To eliminate the carryover ofvelocity from one throttling stage to the adjacent downstream throttlingstage it may be desirable to provide a step as indicated at 39.

FIG. 4 shows a partial sectional view of a plug 3 and seat 3 wherein theplug 3 has three annular skirts 40, 41 and 42 which extend downwardlyand register with three frustoconical surfaces 44, 45 and 46 disposed onthe seat 3. The conical surfaces 44, 45 and 46 form an angle B with aseries of insersecting cylindrical planes represented by the lines 47,48 and 49. The conical surfaces are so disposed that the angle [3increases on consecutive conical surface in downstream directions toprovide increased downstream throat area in the consecutive throttlingstages and to maintain a constant ratio between throat areas as the plug3 lifts from its seat 3'. Thus, the velocities through the consecutivethrottling stages remains generally equal to each other as the plugseparates from its seat.

FIG. 5 shows a partial sectional view ofa plug 4 and seat 4, wherein theplug has two annular skirts 50 and 51, which extend downwardly andregister with two toroidal surfaces 52 and 53. The outer skirt 51registers with the upper convex toroidal surface adjacent a planethrough the center of the tor oid which divides the toroid into twoequal size rings. The inner skirt 50 registers with the convex portionof the toroid adjacent the top of the toroid. The skirts thus cooperatewith the t0- roidal surfaces to provide consecutive throttling stageshaving throat areas which increase in downstream direction and maintainthe same ratio of throat area as the plug 4 separates from its seat 4'.Thus, the arrangement of skirts and toroidal surfaces provide velocitiesthrough the consecutive throttling stages which are generally equal toeach other as the plug 4 separates from its seat 4'. The toroidalsurfaces are so disposed to provide a step 55 between the throttlingstages to prevent carryover of velocity from one stage to the adjacentdownstream stage.

FIG. 6 shows a graphical representation of pressure versus flow for eachplug as it lifts from its seat until it is fully opened. The solid lines1", 2", 3", 4", 5", 6", and 7" represent the pressure versus flow acrossplugs l, 2, 3, 4, 5, 6, and 7, and seats 1', 2', 3', 4', 5', 6', and 7,respectively, as the plugs separate from the mating seats of asequentially operable control valve herebefore made. The dotted curvedlines 1a, lb, 10, 1d, and 1e; 20, 2b, 2c, 2d; 30, 3b, 36; 4a, 4b; 5a,5b; 6a; and 7a represent the values of pressure and flow of steamdownstream of each consecutive throttling stage of the plugs l, 2, 3, 4,5, 6, and 7 and seats 1, 2', 3, 4', 5', 6', and 7 made in accordancewith this invention. a

represents the first throttling stage; b represents the secondconsecutive downstream throttling stage, 0 the third stage and so on foreach plug and seat. As noted hereinbefore, the plugs and seats arenumbered in the sequence in which the plugs lift. Thus a sequentiallyoperable control valve made in accordance with the invention andcontrolling a turbine T supplied with 3,690 psi steam may have, as shownin FIG. 6, five consecutive throttling stages in the first plug 1 andseat 1' to separate, for consecutive throttling stages in the secondplug 2 and seat 2' to separate, three consecutive throttling stages inthe third plug 3 to separate from its seat 3' and two consecutivethrottling stages in the fourth and fifth plugs 4 and 5 to separate fromtheir seats 4 and 5. The sixth and seventh plug 6 and 7 to separate fromtheir seats 6 and 7 may only provide a single throttling. A solid line61 at the top of FIG. 6 represents the steam inlet pressure of 3,690pounds per square inch. Line 63 angled downwardly from line 61represents the steam pressure to the plugs and seats and shows thereduction of pressure with increased flow past the plugs and seats dueto the pressure drop in the inlet piping. A dotted line 64 represents acritical pressure drop, that is the pressure drop across the plug andseat which will produce velocities generally equal to the velocity ofsound in the fluid, Mach 1. It should be noted that the pressure dropacross the first stage a of each plug is generally equal to or less thanthe critical pressure drop, therefore the velocity across the firststage is equal to or less than Mach 1. Since the velocities across theconsecutive stages are generally equal, none of the velocities producedby a valve made in accordance with this invention are greater thanMach 1. Thus, such a sequentially operable control valve will notproduce high intensity noises and vibration.

The consecutive throttling stages in each plug are generally labyrinthtype throttling stages which provide increased throat area in thedownstream direction to produce generally equal velocities across eachstage and generally equal dissipation of energy across each stage asshown by plotting line 65 the pressure drop across the plug 1 and seat 1shown in FIGS. 2 and 3 on the Mollier diagram shown in FIG. 7. Theinitial pressure is 3,690 psi and the first stage of throttling betweenthe frustoconical surface 34 and the lip 25 reduces the pressure to2,050 psi, producing a velocity approximately equal to Mach 1 as theplug begins to lift. However, as soon as the flow rate acrossthe plugand seat increases the velocity decreases below Mach 1. The second stageof throttling between the frustoconical surface 33 and the lip 25reduces the pressure to 1,140 psi and produces a velocity approximatelyequal to Mach 1, as the plug begins to lift, but as before, as the flowrate across the second throttling stage increases the velocity decreasesbelow Mach 1. The third and fourth throttling stages between thefrustoconical surfaces 32 and 31 and lips 25 reduce the pressure to 635psi and 353 psi, respectively, and each throttling stage producesvelocities approximately equal to Mach l, as the plug begins to lift. Asnoted earlier, as rate of steam flowing past the stages increases thevelocity decreases below Mach 1. The fifth throttling stage between theinner skirt 27 and the surface 30 reduces the pressure of the steam to196 psi and also produces velocity approximately equal to Mach 1 as theplug separates from the seat. As steam begins to flow past the plug andseat the velocity reduces to a velocity below Mach 1.

As indicated by the line 65 on the Mollier chart in FIG. 7 thedissipation of energy, change in entropy, is generally equal for eachthrottling stage so that consecutive throttling provides efficientthrottling with a minimum number of stages to handle the pressure dropas the plug begins to separate from its seat and yet provides minimalpressure drop across the plug and seat when the plug is fully opened tooptimize the flow of steam over the plugs and seats at each extreme inthe operation of the sequentially operable control valve. Thus'providinga sequentially operable control valve which is free for high intensitynoises and vibrations when operating at low loads and which has a lowpressure drop when operating near its fully open position.

What is claimed is:

1. A sequentially operable control valve comprising a plurality ofmating plugs and seats, actuating means common to said plugs anddisposed to successively separate said plugs from said seats to allowfluid to flow thcrebetween, said mating plugs and seats having meansdisposed thereon for producing consecutive throttling stages as fluidpasses therebetween, the number of consecutive throttling stages beinggreatest on the first plug and seat to separate, the mating plugs andseats to successively separate having a lesser number of consecutivethrottling stages.

2. A sequentially operable control valve as set forth in claim 1,wherein the means for producing consecutive throttling stages are formedby registering surfaces on said mating plugs and seats cooperating toprovide annular openings which increase in size in downstream direction.

3. A sequentially operable control valve as set forth in claim 1,wherein the means for producing consecutive throttling stages are formedby registering surfaces cooperatively associated to produce annularopenings between plugs and seats and the openings are so disposed thatthe velocities therethrough remain generally equal to each other as theseparation between said plugs and seats increases.

4. A sequentially operable control valve as set forth in claim 1,wherein the means for producing consecutive throttling stages comprisesa plurality of annular skirts disposed on the plug, said skirtsregistering with generally conical surfaces on the mating seat, theslope of the outer conical surface being greater than the slope of theinner conical surface thus providing generally equal velocities betweenthe skirts and conical surfaces as the separation between the matingplug and seat increases.

5. A sequentially operable control valve as set forth in claim 1,wherein the means for producing consecutive throttling stages comprisesa plurality of annular skirts disposed on the plug, at least one skirthaving sloping sides which converge adjacent the mating seat, the matingseat having at least one annular groove, said skirt and groovecooperating to provide a pair of .consecutive throttling stages whichhave openings which increase in size in downstream direction and whichare so disposed that velocities across the openings remain generallyequal to each other as the separation between the mating plug and seatincreases.

6. A control valve as set forth in claim 1, wherein the means forproducing consecutive throttling stages comprises a plurality of annularskirts disposed on at least one plug, said skirts register withgenerally toroidal surfaces on the mating seat, said skirts and toroidalsurfaces being so arranged to provide annular openings through which thevelocities generally remain equal to each other as the separationbetween the mating plugs and seats increases.

7. A sequentially operable control valve as set forth in claim 1,wherein the means for producing consecutive throttling stages comprisesa plurality of concentric annular skirts disposed on the plug, the outerskirts being flared outwardly, the skirts registering with generalytoroidal surfaces, said toroidal surface which registers with said outerskirt being an upper convex portion adjacent a plane through the centerof a toroid and said toroidal surface which registers with the innerskirt being a convex surface adjacent the top of a toroid, said toroidalsurfaces being so disposed to form steps, each step being a throttlingstage and so arranged to provide generally equal velocities through theconsecutive throttling stages as the separation between the mating plugand seat increases.

8. A sequentially operable control valve as set forth in claim 1,wherein the means for producing consecutive throttling stages comprisesa plurality of concentric skirts disposed on the plug, said skirtsregistering with surfaces on the mating seat in such a manner to providelabyrinth type consecutive throttling stages which increase in area indownstream direction and the ratio of the areas remains essentiallyconstant as the plug and mating seat separate.

1. A sequentially operable control valve comprising a plurality ofmating plugs and seats, actuating means common to said plugs anddisposed to successively separate said plugs from said seats to allowfluid to flow therebetween, said mating plugs and seats having meansdisposed thereon for producing consecutive throttling stages as fluidpasses therebetween, the number of consecutive throttling stages beinggreatest on the first plug and seat to separate, the mating plugs andseats to successively separate having a lesser number of consecutivethrottling stages.
 2. A sequentially operable control valve as set forthin claim 1, wherein the means for producing consecutive throttlingstages are formed by registering surfaces on said mating plugs and seatscooperating to provide annular openings which increase in size indownstream direction.
 3. A sequentially operable control valve as setforth in claim 1, wherein the means for producing consecutive throttlingstages are formed by registering surfaces cooperatively associated toproduce annular openings between plugs and seats and the openings are sodisposed that the velocities therethrough remain generally equal to eachother as the separation between said plugs and seats increases.
 4. Asequentially operable control valve as set forth in claim 1, wherein themeans for producing consecutive throttling stages comprises a pluralityof annular skirts disposed on the plug, said skirts registering withgenerally conical surfaces on the mating seat, the slope of the outerconical surface being greater than the slope of the inner conicalsurface thus providing generally equal velocities between the skirts andconical surfaces as the separation between the mating plug and seatincreases.
 5. A sequentially operable control valve as set forth inclaim 1, wherein the means for producing consecutive throttling stagescomprises a plurality of annular skirts disposed on the plug, at leastone skirt having sloping sides which converge adjacent the mating seat,the mating seat having at least one annular groove, said skirt andgroove cooperating to provide a pair of consecutive throttling stageswhich have openings which increase in size in downstream direction andwhich are so disposed that velocities across the openings remaingenerally equal to each other as the separation between the mating plugand seat increases.
 6. A control valve as set forth in claim 1, whereinthe means for producing consecutive throttling stages comprises aplurality of annular skirts disposed on at least one plug, said skirtsregister with generally toroidal surfaces on the mating seat, saidskirts and toroidal surfaces being so arranged to provide annularopenings through which the velocities generally remain equal to eachother as the separation between the mating plugs and seats increases. 7.A sequentially operable control valve as set forth in claim 1, whereinthe means for producing consecutive throttling stages comprises aplurality of concentric annular skirts disposed on the plug, the outerskirts being flared outwardly, the skirts registering with generalytoroidal surfaces, said toroidal surface which registers with said outerskirt being an upper convex portion adjacent a plane through the centerof a toroid and said toroidal surface which registers with the innerskirt being a convex surface adjacent the top of a toroid, said toroidalsurfaces being so disposed to form steps, each step being a throttlingstage and so arranged to provide generally equal velocities through theconsecutive throttling stages as the separation between the mating plugand seat increases.
 8. A sequentially operable control valve as setforth in claim 1, wherein the means for producing consecutive throttlingstages comprises a plurality of concentric skirts disposed on the plug,said skirts registering with surfaces on the mating seat in such amanner to provide labyrinth type consecutive tHrottling stages whichincrease in area in downstream direction and the ratio of the areasremains essentially constant as the plug and mating seat separate.